1. Field of the Invention
The present invention relates to a projection optical system for enlarging and displaying an image on an imager onto a projection plane, and a projection display device incorporated with the projection optical system, and more particularly to an arrangement suitable for use in a projection display device for projecting projection light onto a projection plane in an oblique direction.
2. Disclosure of Related Art
A projection display device (hereinafter, called as a “projector”) for enlarging and projecting an image on an imager such as a liquid crystal panel onto a projection plane (such as a screen) has been commercialized, and widely spread. As an example of the projector of the above type, there has been proposed a projector incorporated with a wide-angle projection optical system, and having an arrangement for tilting a propagating direction of projection light with respect to an optical axis of the projection optical system for tilt projection, in order to shorten a distance between a screen and a projector main body.
For instance, there is known a projector incorporated with a wide-angle lens having a large angle of view, as a projection optical system. In the projector, the throw distance can be shortened, and tilt projection free of distortion is realized by shifting an imager and a screen in directions away from each other with respect to an optical axis of the projection optical system. The projector, however, requires a wide-angle lens having a large angle of view. Accordingly, the size of the projector main body may be increased, as the size of the lens is increased.
There is proposed a projector incorporated with a projection lens unit and a mirror, as a projection optical system. In the projector, the throw distance can be shortened by: forming an image on an imager, as an intermediate image, at a position between the projection lens unit and the mirror; and enlarging and projecting the intermediate image by the mirror. According to the projector, a wide angle projection is realized with use of a mirror having a relatively small curved surface. This enables to suppress increasing the size of the projector main body.
In the case where a device incorporated with a projection lens system and a reflection mirror, as recited in the latter arrangement, is fabricated as a product, the device may have an arrangement as shown in e.g. FIG. 6.
Specifically, an optical engine 510a is housed in a casing 510, and the optical engine 510a is operable to generate light (hereinafter, called as “modulated image light”) which is modulated in accordance with an image signal. An imager for generating modulated image light is disposed at such a position that a center of an effective display area of the imager is displaced on the upper side with respect to an optical axis L1 of a lens unit 520, in other words, on the side of a window 540a formed in an upper surface of a cover member 540, by a predetermined distance “d”. Modulated image light through the imager is incident onto the lens unit 520. The modulated image light through the lens unit 520 is convergently reflected on a reflection mirror 530.
The reflection mirror 530 has a concave reflecting surface of an aspherical shape or a free curve shape. The reflection mirror 530 is disposed on the lower side (opposite to the window 540a) with respect to the optical axis L1 of the lens unit 520. Modulated image light reflected on the reflection mirror 530 is transmitted through the window 540a formed in the upper surface of the cover member 540, and projected onto a screen plane as wide-angle light.
In the above arrangement, as shown in FIG. 6, a portion of the cover member 540, corresponding to the reflection mirror 530, is projected downwardly by a significant length. Accordingly, a large step portion G is likely to be defined between a lower end of the projecting portion, and a bottom surface of the lens unit 520. A leg portion 550 is formed on a bottom surface of the casing 510 or a like member to compensate for the step portion G. However, the projector having the above arrangement is likely to be unstably sit, when being sit on the table as shown in FIG. 6.